The static latent image developing method represented by an electrophotographic method is widely applied as method forming means such as a printer, a copy machine and a facsimile machine.
Because the method is a highly accomplished method by which a high quality image can be stably obtained at a high speed. However, some problems are remained.
For example, in usual toner produced by a crushing method, the materials dispersed in the toner are not uniformly distributed at the broken surface and the surface properties of each toner particles are difficultly made the same. Therefore, fluctuation of toner transfer in the transfer process tens to be occurred so as to lower the color reproducibility in a color image.
The toner for developing static latent image is required to reduce the particle diameter for raising the image quality. Recently, the toner produced by a polymerization method is actively developed as the method for producing the toner having a small particle diameter. Examples of such the method include a method for producing a toner particle having an irregular shape by salting, coagulation or adhering by fusion of a resin particle and a colorant particle, and a method by which a radical polymerizable monomer and a colorant are dispersed in an aqueous medium so as to form a liquid droplet having the designated toner diameter and subjected to suspension polymerization.
The toner particle having a spherical shape and uniform surface property can be produced by the suspension polymerization method. Therefore, the uniformity of the toner particles is raised but the adhesiveness to the image carrier is made higher since the shape of the toner is sphere. Accordingly the transfer ability of the toner particle is lowered.
A non-spherical particle produced by a method is disclosed in Japanese Patent Publication Open to Public Inspection, hereinafter referred to as JP O.P.I. Publication, No. 11-194540, in which a resin particle is prepared by polymerized in an aqueous medium containing a surfactant and treated by a coagulating agent in a concentration more than the critical coagulation concentration of the resin particle and a water-miscible organic solvent.
In the foregoing technology, a toner excellent in high uniformity of the shape and the charging amount and high sharpness of the image can be obtained by the use of two- or three-valent metal salt. However, the craft point of the surfactant is raised by the presence of the two- or three-valent metal salt so as to form a precipitation difficultly soluble in water. The precipitation is remained in a state of adhered to the toner particle and tends to cause lowering of the fine line reproducibility and spreading of the character image.